1. Field of the Invention
The present invention relates to a novel method of olefin polymerization which allows for the production of multimodal polyolefins of tunable composition, molecular weight, and polydispersity.
2. Background Art
Polymeric materials that are multimodal in molecular weight distribution (MWD), composition, or microstructure have distinct advantages over the corresponding traditional monomodal materials. And while multimodal materials hold much promise for advances in polymer chemistry, the problems associated with their preparation have limited their use.
Homogenous high molecular weight (HMW) polyethylene has promise in many applications. It combines low cost with good mechanical properties such as high strength and durability. This material, however, is difficult to process and mold due to its high melt-temperature and poor Theological properties. To solve this problem, HMW polyethylene is formulated with plasticizers to increase its processability. The use of plasticizers drives up the cost of producing the material and limits its utility because of phase separation problems. Polyethylene that is multimodal in molecular weight inherently has a lower melt transition temperature and improved Theological properties that makes it easier to process. Because the composition contains no additives, there is no problem with phase-separation. Importantly, multimodal polyethylene maintains the same mechanical properties of the HMW material.
The preparation of multimodal polyolefins has been problematic, thus limiting their application. Mixed catalyst systems have been used to prepare materials that have broad MWD but the process is difficult to control and the outcome difficult to predict. Another method to prepare polymers of broad or multimodal MWD utilizes separate reactors or blends of separate polymers of different MWD. These methods increase production costs and are often plagued by incomplete mixing, which can give rise to polymer phase separation. A single-process method that is reliable, cost-effective, and predictable for preparing multimodal homogeneous polyolefins is needed.
Block copolymers with multimodal MWD, in particular, hold great technological promise. These materials may combine the fluictionality of two distinctly different polymer microstructures with the advantages of a multimodal MWD. Hybrid materials, such as thermoplastic elastomers, maintain good mechanical properties and good processability.
A particularly useful class of block copolymers is the stereoblock polyolefins. One method for their preparation is disclosed in J. C. Chien, et al., Macromolecules 30:3447–3458 (1997), and relies upon the use of mixtures of zirconocene catalysts that manifest different stereoselectivities during propagation (e.g., isotactic and atactic) so that a stereoblock microstructure will result. See also: J. C. Chien, et al., J. Polym. Sci. Part A 37:2439–2445 (1999); C. Przybyla, et al., Acta Polym. 50:77–83 (1999); and S. Lieber and H. H. Brintzinger, Macromolecules 33:9192–9199 (2000). These methods do not, however, lead to multimodal materials. These methods further suffer from the same drawbacks mentioned above when using mixed catalyst systems. A single-process method that is reliable, cost-effective and predictable for preparing multimodal stereoblock polyolefins is needed.